This invention relates to methods of fabricating integrated circuits; and more particularly, it relates to the steps in such method that determine whether or not the substrate on which the integrated circuits are being fabricated contains gettering sites for mobile impurities.
Ideally, the semiconductor substrate on which an integrated circuit is fabricated is made of pure silicon except for a very small amount of P type or N type dopant atoms such as boron or phosphorus. However, in an actual substrate, various other elements are also present in trace quantities as undesirable impurities. For example, traces of copper and iron are typically present.
Such impurities if not dealt with cause the integrated circuit to malfunction. Typically, the impurities ionize and are mobile; and thus they produce undesired currents within the circuit during its operation. To avoid these currents, gettering sites are introduced into the backside of the substrate; and the integrated circuit is then fabricated on the opposite or frontside of the substrate.
A common way to introduce gettering sites into the substrate is to sandpaper the backside surface. This produces a mechanical stress or defects in the substrate's crystal lattice. In turn, this has the effect of creating new allowable energy levels between the conduction band and valance band of the crystal; and these new energy levels act as traps for the mobile impurity ions in the substrate. When a mobile impurity ion migrates in the substrate to a position near the gettering sites, it gets trapped in the new energy levels and thus stops migrating. Consequently, the undesired currents in the circuit stop.
A problem, however, is that heretofore the presence or absence of the gettering sites in the substrate has been difficult if not impossible to detect until after the integrated circuit has been fabricated and tested. Surface texture of the backside surface of a substrate is not a measure of the degree of gettering sites present. A substrate having a rough backside surface can have none or many gettering sites; and the same is true for a substrate having a smooth backside surface.
Consequently, if a purchaser buys semiconductor substrates that are supposed to contain gettering sites, but in fact they do not, the purchaser will not know this until after he fabricates his circuits on the substrates and discovers they fail. But the fabrication of circuitry on a substrate is an expensive and time consuming process.
Accordingly, a primary object of the invention is to provide a method of fabricating integrated circuits which incorporates novel steps for detecting the presence or absence of gettering sites in a substrate.